Hard to Put Red-Light Violations Under a Lens

As red-light cameras have proliferated around the U.S. over the past two decades to hundreds of cities and towns, there is one troubling detail: They don't always make traffic intersections safer.

Police departments in more than 500 cities and towns use the cameras—and, usually, signs warning of their presence—to record motorists who run red lights and to ticket them. The goals are to deter drivers from going through an intersection...